1. Field of the Invention
The present invention relates to an ink jet printing apparatus with a protective mechanism for preventing a clogging of nozzles and ink ejection anomalies in a print head.
2. Description of the Related Art
As recording apparatus (or printing apparatus) with a function of printer, copying machine or facsimile, or as printing apparatus used as output devices for composite electronic devices including computers and word processors as well as for workstations, ink jet printing apparatus, which execute printing by ejecting ink onto a print medium (print material), such as paper, cloth, plastic sheet and OHP sheet, according to image information (print information), have become more widespread.
There are growing demands that printing apparatus print on a variety of print materials. To meet these demands, development efforts have been made in recent years and printing apparatus are now available which use as print media cloth, leather, nonwoven cloth and metals in addition to ordinary print media such as paper (including thin paper and processed sheet) and thin resin sheet (OHP sheet).
The ink jet printing apparatus, which ejects liquid ink from a print head as a printing means onto a print medium to form an image thereon (i.e., performs printing), has many features, such as an ease with which the apparatus size can be reduced and an ability to print a full color image. There is a trend in recent years for an increased number of ink colors (ink kinds) used, as a result of adding light-colored inks for reducing a graininess and special color inks for widening a color reproduction range in addition to four colors, cyan, magenta, yellow and black, the minimum required for full color printing. It has also been proposed to use a plurality of print heads for each color to increase the printing speed. Against this technical background in recent years, the number of print heads mounted on the ink jet printing apparatus has tended to increase.
A nozzle face of the print head is normally covered with a cap when a printing operation is not performed because directions and volumes of ink ejections are affected by dirt adhering to ink ejection openings or by ink near the nozzle openings being evaporated. Further, various recovery mechanisms for recovering a normal state of the print head are provided, which include a suction mechanism that sucks out viscous ink from the nozzle openings in a capped state and a preliminary ejection mechanism that performs preliminary ejections (also referred to as “idle ejections”) in the capped state. The number of capping means including the cap and the recovery mechanisms tend to increase with the increasing number of the print heads used.
As the number of capping means increases, another construction is proposed in which, rather than providing all the caps with the suction mechanism, the suction mechanism is used on only a part of the caps, with the remaining caps serving as protective caps that cover the nozzle faces of the print heads (Japanese Patent Application Laid-open No. 7-032599 (1995)). For suction operation, each of the print heads is moved to the suction cap that has the suction mechanism. By setting the number of suction caps less than the number of the print heads, the cost and size of the printing apparatus can be reduced.
With this construction, however, while the suction cap maintains a proper level of moisture in it at all times by the suction of ink, the interior of the protective cap with no suction mechanism is not moist enough to prevent the nozzle face from becoming dry It is therefore difficult to maintain a highly reliable ejection performance for the nozzles capped with the protective cap.
Since the ink jet printing apparatus perform printing by ejecting ink from minute nozzles, the nozzles may get clogged with ink, resulting in ink ejection anomalies (including ejection failures) and therefore a degraded quality of printed image. To prevent this some ink jet printing apparatus perform an ejection recovery operation to maintain or reinstate a normal ink ejection performance of the printing means. Two examples of such ejection recovery operations are described below.
A first example involves ejecting ink from all nozzles of the print head onto an area outside an image forming area (referred to as an “idle ejection operation”) to prevent possible ejection failures caused by an increased ink viscosity due to characteristic changes with the passage of time.
A second example involves providing the print head protective capping means with a forced flow producing means, such as a pump, to cause ink present near the nozzle openings in all print heads to flow in order to remove viscous ink, which is formed after the apparatus has been left idle for a long period of time, from near the nozzle openings and to remove dirt or foreign substances that adhere to the nozzle openings and may cause ink ejection failures.
In conventional practice, these recovery operations have been executed periodically to keep the nozzle face clean.
Adopting the above-described first and second method in the ink jet printing apparatus, which has only a part of the caps function as a suction cap, to perform the similar ejection recovery operation on all print heads, however, has the following problems.
With the first method, because a protective cap with no suction mechanism is not moist inside, a print head protected by the protective cap when not used has an ink viscosity change faster than does the print head protected by a suction cap with the suction mechanism, and therefore requires idle ejections prior to printing even after a short period of rest. If all the print heads protected by the protective caps are to perform idle ejections required after a short period of rest, this results in the print head protected by the suction cap performing the idle ejections too often and therefore wasting ink through more than necessary idle ejections.
As to the second method, because the protective cap as described above is not moist inside, a print head protected by the protective cap when not used has an ink viscosity change faster than does the print head protected by a suction cap with the suction mechanism. If printing is to be performed after the print head has been left idle for a longer period of time than in the first method, a print head protected by a protective cap may not be able to eject ink normally unless a forced ink flow is produced in the print head prior to the start of printing operation. On the other hand, a print head protected by a suction cap may be able to eject ink normally without having to cause a forced ink flow in the print head. Therefore, if the similar forced ink flow is produced in all the print heads, the print head protected by the suction cap will have a time loss due to the unnecessary, forced ink flow and an increased volume of waste ink.